U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications

Variability in the Eastern Equatorial Pacific Ocean During 1986-1988

Michael J. McPhaden and Stanley P. Hayes

NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington

Journal of Geophysical Research, 95(C8), 13,195-13,208 (1990)
Not subject to U.S. copyright. Published in 1990 by the American Geophysical Union.

5. Basin Scale Forcing

In the preceding section it was suggested that much of the variability observed in the eastern equatorial Pacific during 1986-1988 could not be explained solely in terms of local wind forcing. We explore this issue in more detail by examining time series of wind stress from six locations along the equator spanning 110W to 165E. Stress has been calculated assuming a drag coefficient of 1.2 10 [Large and Pond, 1981] and air density of 1.2 kg m. The mooring wind time series have been adjusted to 10 m anemometer height assuming a neutral stability, constant stress layer.

Figure 8 shows the zonal wind stress component overplotted on the mooring climatologies (110W and 140W) and the Wyrtki and Meyers [1975] stress climatology (165E, Nauru, Baker, and Christmas islands). The 165E time series (which is also plotted in Figure 3) shows westerly or anomalously weak easterly winds for most of mid-1986 to early 1988. This is followed by anomalously strong easterly wind stresses of up to 0.05 N m during March-December 1988. The winds in 1986-1987 are also frequently punctuated by episodes of strong westerlies lasting 10 days to 2 months, most notably in May 1986, November-December 1986, and June-July 1987. Variations on both these intraseasonal time scales and on interannual time scales exceed those for the mean seasonal cycle, which has a range of only 0.03 N m at this location.

Figure 8. Time series of zonal wind stress at various locations in the equatorial Pacific. Data at 165E are from 0 (solid line) and 2S (dashed line). Superimposed are estimates of the monthly mean seasonal cycle from Wyrtki atid Meyers [1975] for Nauru, the moorings at 165E, Baker, and Christmas. Mooring mean seasonal cycles are overplotted on the 140W and 110W time series. Daily data have been smoothed with an 11-day Hanning filter.

The remote response to the May 1986 wind burst has been discussed by Miller et al. [1988], McPhaden et al. [1988, 1989], and Harrison and Giese [1989]. The May westerly wind burst excited a Kelvin wave which could be traced all the way to the South American coast. Its signature may be seen in the dynamic height time series in Figure 3, in the current time series in Figure 4, and in the transport time series in Figure 5. Eastward propagating pulses in dynamic height appear at other times as well, as, for example, following the November-December 1986 and June-July 1987 westerly wind events. In each case the time lag between the maximum wind stress and maximum dynamic height at 140W implies a zonal phase speed of approximately 2-3 m s, comparable to first baroclinic mode Kelvin wave phase speeds along the equator.

Figure 8 shows that pronounced westerly wind events are confined to west of the date line. However, lower-frequency interannual variations are not. In particular, a weakening of the easterlies can be seen as far west as 140W in 1987, and stronger than normal easterlies prevail between 165E and 157W in 1988. To highlight the large-scale interannual variations, we computed monthly averaged wind anomalies in both the meridional and zonal directions and then smoothed them with a 1-2-1 filter in time. The results are shown in Figure 9. As discussed in Appendix B, the Wyrtki and Meyers [1975] climatology is weaker at 110W and 140W than the mooring climatologies. Hence, if Figure 9 were plotted using the Wyrtki and Meyers stress climatology at all locations, 110W and 140W would show stronger easterly anomalies and weaker westerly anomalies. Also recall that westerly winds are underestimated at Nauru because of topographic shielding.

Figure 9. Monthly mean vector wind stress anomalies corresponding to the time series in Figure 8.

Figure 9 shows that easterlies were stronger than normal by about 0.01-0.02 N m between 167E and 157W in early 1986. Then in mid-1986, westerly anomalies of similar magnitude developed at 165E, Nauru, and Baker islands; at 165E these anomalies intensified to nearly 0.05 N m during the November-December 1986 westerly wind event. During 1987, westerly anomalies of 0.02 N m could be found between 165E and 140W, the largest of which (0.05 N m) occurred between 165E and 176W. Westerly anomalies diminished in late 1987 and were replaced by easterly anomalies which persisted until the end of 1988 between 165E and 157W.

It is interesting that the largest zonal wind stress anomalies at 110W tend to be directed opposite to those in the central Pacific during 1987-1988. Specifically, when the winds are anomalously westerly (easterly) in 1987 (1988) at Nauru, Christmas, and Baker islands, the winds at 110W are anomalously easterly (westerly). The distribution of zonal winds along the equator in 1987 is similar to that documented by Rasmusson and Carpenter [1982] during the mature phase of their composite El Nio. These zonal variations in this wind field may be related to east-west displacements of the "Walker cell," a thermally direct atmospheric circulation on the equatorial place driven by SST-induced heating [Gill, 1982]. A detailed analysis of the relationship between surface winds, SST, and atmospheric heating is beyond the scope of this study, however.

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